This study will examine the specificity and significance of retrograde axoplasmic transport to the visual system, using biochemical and light and electron microscopic cytochemistry. Purified isoenzymes of horseradish peroxidase will be modified chemically and tested in vivo to determine which groups on this molecule are required for its uptake and retrograde transport by axons of ganglion cells of the sensory trigeminal ganglion which innervate the rabbit cornea.